Heat exchangers such as steam generators in a pressurized water nuclear reactor incorporate a tube bundle consisting of a very large number of small-diameter tubes arranged vertically inside a bundle casing of a generally cylindrical shape, itself arranged inside the thick rigid outer pressurized casing of the steam generator.
Pressurized water of the nuclear reactor circulates inside the small-diameter tubes and the water to be vaporized is introduced into the bundle casing, where it comes into contact with the outer surface of the tubes. Steam is collected at the top part of the bundle casing and it is then generally dried in steam-water separators arranged in the outer casing of the steam generator above the bundle casing.
The steam generator is fed with water to be vaporized by a means which is generally situated near the top part of the bundle casing. Furthermore, water recovered from the steam by the water-steam separators is also sent back to the bundle casing to be vaporized. A continuous water circulation is thus produced inside the steam generator casing. This circulation may require channelling means such as additional cylindrical casings arranged coaxially with the bundle casing and with the outer casing of the steam generator, solely over a part of their side surface.
The steam generators are very tall units in which the various coaxial casings are arranged with a radial spacing which is generally small.